It seems a long time ago now, but once airlines were not even seen as investable by a large portion of money managers. The businesses were highly capital intensive and deeply competitive, producing major swings in profitability – from modestly profitable in good times to deep losses in bad times. United and American airlines have both been in bankruptcy protection in the last twenty years.

Following the financial crisis, however, a new sense of discipline seemed to emerge in the industry and investors returned to the sector. Today, Delta carries a market cap of $43 billion; with American at $28 billion and United at $23 billion. Mergers in the last decade have shrunk capacity growth and instilled discipline to the industry.

Laying out its plans over the next several years after it announced earnings on January 23rd, United may have shattered some of the new found confidence in the United States’ airline industry. United said that it would expand its capacity, which could suggest a more competitive pricing environment between it and its two main competitors, Delta and American. United is targeting “mid-continental hubs” where it is relatively weaker to complement its strength in key international destinations – typically the largest airports. Prices tend to be higher outside the nation’s largest airports where competition is most intense.

United is now just the fourth largest airline in the United States, with about 14% of airfares, trailing Southwest, American, and Delta. It’s share of domestic flights, meanwhile, is only about 10%. According to United, revenue per available seat mile at the mid-market locations is about 10.5 cents versus less than 9 cents at the large airports where United is stronger in transporting international customers.

Fears of a more aggressive United leading to weaker pricing across the board has sent United shares down 5% in after-hours trading, while Delta is down 3.5% and American is down 4%.

Meanwhile, United’s earnings for the previous quarter were actually quite good. Revenue increased by 4% and EPS at $1.40 was $.06 higher than analyst estimates.